3. Ageratum conyzoides Linnaeus, Sp. Pl. 2: 839. 1753.
Ageratum latifolium Cavanilles
Annuals, perennials, or sub-shrubs, 20–150 cm (fibrous-rooted). Stems erect, sparsely to densely villous. Leaf blades ovate to elliptic-oblong, 2–8 × 1–5 cm, margins toothed, abaxial faces sparsely pilose and gland-dotted. Peduncles minutely puberulent and sparsely to densely pilose, eglandular. Involucres 3–3.5 × 4–5 mm. Phyllaries oblong-lanceolate (0.8–1.2 mm wide), glabrous or sparsely pilose (margins often ciliate), eglandular, tips abruptly tapering, subulate, 0.5–1 mm. Corollas usually blue to lavender, sometimes white. Cypselae sparsely strigoso-hispidulous; pappi usually of scales 0.5–1.5(–3) mm, sometimes with tapering setae, rarely 0. 2n = 20, 40.
Flowering Jul–Aug. Disturbed sites, mostly coastal; 0–20 m; introduced; Ala., Calif., Conn., Fla., Ga., Ky., Md., Miss., Mo., N.C.; South America; introduced, Mexico; West Indies; Central America; Pacific Islands (Hawaii).
Ageratum conyzoides is apparently native to South America. North American plants were escapes and naturalized from cultivation.